Structural Characterization of Rare-Earth Doped Yttrium Aluminoborate Laser Glasses Using Solid State NMR

The structure of laser glasses in the system (B2O3)0.6{(Al2O3)0.4−x (Y2O3) x } (0.1 ≤ x ≤ 0.25) has been investigated by means of 11B, 27Al, and 89Y solid state NMR as well as Y-3d core-level X-ray photoelectron spectroscopy. 11B magic-angle spinning (MAS) NMR spectra reveal that the majority of the boron atoms are three-coordinated, and a slight increase of four-coordinated boron content with increasing x can be noticed. 27Al MAS NMR spectra show that the alumina species are present in the coordination states four, five and six. All of them are in intimate contact with both the three- and the four-coordinate boron species and vice versa, as indicated by 11B/27Al rotational echo double resonance (REDOR) data. These results are consistent with the formation of a homogeneous, nonsegregated glass structure. For the first time, 89Y solid state NMR has been used to probe the local environment of Y3+ ions in a glass-forming system. The intrinsic sensitivity problem associated with 89Y NMR has been overcome by combining the benefits of paramagnetic doping with those of signal accumulation via Carr-Purcell spin echo trains. Both the 89Y chemical shifts and the Y-3d core level binding energies are found to be rather sensitive to the yttrium bonding state and reveal that the bonding properties of the yttrium atoms in these glasses are similar to those found in the model compounds YBO3 and YAl3(BO3)4. Based on charge balance considerations as well as 11B NMR line shape analyses, the dominant borate species are concluded to be meta- and pyroborate anions.